The present invention relates to a filter or tool-changing device for extrusion devices and more particularly, for screw extrusion devices used for extruding plastics material. Known filter-changing devices comprise a filter carrier which is displaceable perpendicularly to the direction of flow of the moulding composition to be filtered. The filter carrier has filter inserts received therein. The filter carrier may be in the form of a flat slider member which accommodates a plurality of filter inserts adjacent to one another or may be a plurality of separate filter-carrier members, each of which accommodates only a single filter insert. The filter-carrier must be secured in position when the filter inserts are being used and it must also be reliably sealed so that none of the plastics material can leak therepast.
It is known from United States Patent Specification No. 3,684,419 to provide an annular pressure element for securing and sealing the filter-carrier in a filter housing of a filter-changing device. This pressure element is displaceable in an annular cylindrical chamber and is hydraulically actuated. A sealing ring or washer is mounted in front (downstream) of the pressure element and provides the necessary sealing by acting upon the pressure element which is pushed against the periphery of the filter insert. After the hydraulic pressure has been reduced, the filter-carrier may be quickly detached and displaced. After the filter-changing process has been concluded, the hydraulic pressure is again increased to its original level so as to secure and seal the filter-carrier in the operating position, thereby securing and sealing the new filter insert in its correct position.
One disadvantage of such an arrangement is that an annular piston, which is displaceable in the cylindrical chamber, must have a number of piston seals provided on its external and internal surfaces. These seals require regular maintenance and may leak, thereby disrupting the extrusion process.
In addition, if a molten mass of plastics material is being processed, it is not possible to use conventional elastomeric sealing rings. This is because the plastics material may be at a temperature which is sufficiently high to cause damage to the rings.
A further filter-changing device is known from German Offenlegungsschrift No. 2 903 773 in which the annular pressure chamber has a front end portion in the form of a membrane or diaphragm. The membrane is deformable in an axial direction and abuts the filter-carrier, under the effect of the pressure in the pressure chamber, around the periphery of the filter insert. The filter-carrier should therefore be secured and sealed from the filter housing.
The membrane used in such annular pressure chamber is relatively thin so as to permit a possible lifting movement if pressure is applied to the filter-carrier. It will be apparent that a relatively thin membrane is subjected to some friction during the change-over of the filter-carrier and is therefore subject to wear. Consequently, the service life of the membrane is short. In addition, the membrane cannot compensate for thermal expansion of the filter housing nor for manufacturing tolerances in the dimensions of filter device, because the lifting capacity of the membrane acting in opposition to an annular piston is very small.
The present invention seeks to provide a filter-changing device in which the hydraulic pressure medium present in the annular pressure chamber is adequately sealed and in which the annular pressure element has an adequate lifting capacity.
According to the present invention, there is provided a filter or tool-changing device for screw extruders utilised for extruding plastics material, comprising a filter or tool-carrier which is displaceable perpendicular to the direction of flow of the plastics material, the carrier being secured and sealed in its operating position within a housing by an annular pressure element which is hydraulically actuatable by means of hydraulic fluid present in an annular pressure chamber, wherein the annular pressure chamber is defined by two metal bellows which are disposed concentrically one with the other, one end of each bellows being mounted on the annular pressure element and the other end of each bellows being mounted on the housing.
Because of their design, the metal bellows have a large extension capacity. The lifting movement of the pressure element, because it is connected to the two bellows, is substantially longer than in most, if not all, known devices. Sealing problems with regard to the hydraulic pressure medium do not arise, because the folding bellows are fixedly connected to the annular pressure element and to the housing by, for example, welding. Furthermore, the specific stroke length utilising metal folding bellows requires less force to be applied than would be the case if a metal membrane were used on the front end of the pressure chamber.
In an advantageous embodiment of the invention, an annular displacement member protrudes into the annular pressure chamber. In this way, the volume of the annular pressure chamber is reduced so that a smaller quantity of hydraulic fluid is needed. The thermal expansion of the hydraulic pressure medium is therefore reduced, so that the compensation tank, necessary in hydraulic systems, can be reduced in size.
The invention therefore provides a filter or tool-changing device utilising an annular, hydraulically actuated, piston of simple construction and in which satisfactory sealing of the hydraulic system and simultaneously, an adequate, axial stroke length are produced.